Deacidizing gaseous effluents such as, for example, natural gas and combustion fumes is generally carried out by washing with an absorbent solution. The absorbent solution allows to absorb the acid compounds present in the gaseous effluent (H2S, mercaptans, CO2, COS, SO2, CS2).
Deacidizing these effluents, notably decarbonation and desulfurization, imposes specific constraints on the absorbent solution, in particular thermal and chemical stability, notably to the effluent impurities, i.e. essentially oxygen, SOx and NOx.
The most commonly used absorbent solutions are aqueous alkanolamine solutions. Document FR-2,820,430, which provides deacidizing methods for gaseous effluents, can be mentioned.
However, it is well known to the person skilled in the art that these amines have the drawback of degrading under the conditions of use.
In particular, amines can be degraded by oxygen forming acids such as, for example, formic acid, acetic acid or oxalic acid in amine solutions.
These acids react with the amines according to an acid-base reaction and form salts referred to as Heat Stable Salts (HSS) or Heat Stable Amine Salts (HSAS). These acids are stronger acids than carbonic acid (formed by the reaction of CO2 with water). The salts they form by reaction with the amines are thus not regenerated in the regeneration column under normal unit operating conditions and they accumulate in the unit.
In the case of CO2 capture in fumes from industrial units, electricity or more generally energy production units, amine-containing absorbent solution degradation phenomena are increased by the presence of a massive amount of oxygen in the feed to be treated, up to 5 vol. % in general. In the case of fumes from natural gas combined cycles, the volume proportion of oxygen in the fumes can reach 15%.
The degraded solution is characterized by:
a decrease in the absorption of the acid compounds of the feed in relation to a fresh amine solution,
an increase in the density of the absorbent solution and in its viscosity, which can lead to a performance loss,
the formation of more volatile amines that pollute the treated gas and the acid gas from the regeneration stage: ammonia, methylamine, dimethylamine and trimethylamine, for example, depending on the nature of the amine used,
an accumulation of degradation products in the absorbent solution, which can require treating the degraded solution,
possible foaming problems due to the degradation products.
The degradation of the absorbent solution thus penalizes the performances and the smooth running of gas deacidizing units.
In order to overcome the degradation problem, for lack of limiting or suppressing the presence of oxygen in the absorbent solution, compounds whose purpose is to prevent or to limit amine compounds degradation, notably the degradation generated by oxidation phenomena, are added to the absorbent solution. These compounds are commonly referred to as degradation inhibiting agents. The main known modes of action of degradation inhibiting agents are, depending on their nature, a reaction of reduction type and/or capture, trapping and/or stabilization of the radicals formed in the absorbent solution in order to limit or to prevent, or to stop, the degradation reactions, notably chain reactions.
U.S. Pat. No. 5,686,016 and U.S. Pat. No. 7,056,482 mention additives used for limiting the degradation of absorbent solutions respectively used for natural gas deacidizing and CO2 capture.